The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing printed media. Hewlett-Packard's contributions to this technology are described, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992), and Vol. 45, No. 1 (February 1994).
Generally, an ink-jet image is formed when a precise pattern of dots is ejected from a drop generating device known as a "print head" onto a printing medium. The typical ink-jet print head has an array of precisely formed nozzles in an orifice plate that is attached to an ink barrier layer on a thermal ink-jet print head substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorant dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a "firing resistor", located opposite each nozzle so ink can collect between the firing resistor and the nozzle. When electric printing pulses heat the thermal ink-jet firing resistor, a small volume of ink adjacent the firing resistor is heated, vaporizing a bubble of ink, and thereby ejecting a drop of ink from the print head. The droplets strike the printing medium and then dry to form "dots" that, when viewed together, form the printed image.
The physical arrangement of orifice plate, ink barrier layer, print head substrate, and various intermediate layers on the substrate is further described and illustrated at page 44 of the Hewlett-Packard Journal of February 1994, cited above.
In ink-jet print head technology the orifice plate is expected to be permanently attached to the ink barrier layer on the print head substrate. Delamination of the interface between the orifice plate and the barrier layer has always been a problem but recently the problem has increased in significance.
Delamination principally occurs from environmental moisture and the ink itself. Environmental moisture develops from storing the print cartridge in a capping station on the printer, in normal, open room storage, or in shipping packages. Environmental moisture has become an increasing problem because print cartridges are increasingly being subjected to longer and longer periods of storage. As for ink, it has become a problem because some inks wick much more into the interface between the orifice plate and the barrier. Such inks contain surfactants and solvents that increase the capillary effect at the orifice plate-ink barrier interface.
Delamination of the orifice plate is manifested in several ways. Full delamination occurs when the orifice plate falls off the print cartridge. The print cartridge deprimes, and the electrical leads within the printer can be shorted out. When partial delamination occurs, print cartridge performance and print quality can degrade markedly. Delamination changes the architecture of the ink conduits and firing chambers. Fluidic isolation of the firing chambers can be lost, cross-talk between the firing chambers and ink conduits can develop, and if there are inks of different colors in adjacent chambers, color mixing can occur.
U.S. Pat. No. 5,493,320 entitled "Ink-jet Printing Nozzle Array Bonded to a Polymer Ink Barrier Layer" by Sandbach et al. issued on Feb. 20, 1996 recognizes the problem of orifice plate-ink barrier layer delamination. This patent, however, does not go far enough and does not contemplate the measures needed to be taken against very aggressive inks and increased storage times in printers, open room environment, or shipping packages.
It will be apparent from the foregoing that although there are many varieties of print cartridges and processes for making them, there is still a need for an approach that avoids both full and partial delamination of the orifice plate-ink barrier layer interface.